Although stroke is the fourth leading cause of death in the U.S., the current treatment strategy of thrombolysis for the majority of strokes due to a blood clot (ischemic) has mixed results. Thus, there is an urgent need for new and better stroke therapies. We have previously demonstrated that the domain V protein fragment of the brain extracellular matrix component perlecan is both neuroprotective and neuroreparative after ischemic stroke by interaction with the brain endothelial cell receptor ?5?1 integrin. Intriguingy, preliminary results now suggest that mice with an endothelial selective deletion of ?5?1 integrin are profoundly resistant to experimental ischemic stroke; they show little to no signs of brain injury. Furthermore, inhibition of the ?5?1 integrin with ATN-161 or its more potent isoform Ac-PhScN-NH2 conveys nearly identical resistance to stroke injury in wild type mice. This may occur via stabilization of the blood-brain barrier through increased function of the brain endothelial cell tight junction protein claudin-5, which in turn minimizes vasogenic edema, inflammation, and injury. Therefore, we hypothesize that endothelial cell ?5?1 integrin could be a particularly effective therapeutic target for stroke. In this application, we propose the followig specific aims: 1. Determine the effect of endothelial cell selective ?5?1 integrin deletion on experimental ischemic stroke, 2. Determine the potential of the ?5?1 integrin as a therapeutic target in experimental ischemic stroke and 3. Determine, in mechanistic detail, the role of ?5?1 integrin in modulating blood-brain barrier integrity and subsequent resistance to ischemic stroke. We will use several novel genetically modified mice (?5 integrin endothelial cell specific knockdown mice, claudin5 eGFP tagged mice) and newly characterized ?5?1 integrin inhibitors in experimental stroke models and in vitro endothelial cell barrier assays. We expect to demonstrate that suppression or inhibition of ?5?1 integrin in endothelial cells affords significnt blood-brain barrier-mediated resistance to experimental ischemic stroke, supporting our long-term goal of developing ?5?1 integrin as a novel human stroke therapeutic target.